Structure Regulation And Electrochemical Performance Of Carbon Coupled Nickel-Based Materials

Supercapacitors involving the surface electrochemical reactions hold great promise in the field of energy storage and conversion,due to the high specific capacity,safety in use and environmentally friendly.However,the slow ion transportation and electron mobility result in severe specific capability loss at high current density,and further hinder the increment of rate capability and long-term cycling performance.In this thesis,we have proposed to improve the ion transportation and electron transfer in the electrode by increasing the interaction between active materials and substrate,optimizing the composition,coordination environment and strain in the materials,thus,lead to the enhancement of rate capability and electrochemical stability.By employing the carbon with high conductivity as substrate,we developed a series of carbon coupled Ni-based nanomaterials by regulating the surface properties of carbon-based substrate and the composition of Ni-based active species.We have revealed the in-situ electrochemical phase transition mechanism and the correlation between the multiscale structure and electrochemical performance for the electrode materials.The main results are summarized below.NiCo-layered double hydroxide（NiCo-LDH）nanosheets have been grown on the superhydrophilic carbon cloth in a vertical orientation（CC-NC-LDH）,by employing the nitrogen-doped carbon layer as the structure/interface coupling bridge and followed by one-step hydrothermal method.The nitrogen-doped carbon layer not only improves the hydrophilicity of the carbon surface,confirmed by a sharp decrease in water contact angle from 135° to 0°,but also enhances the interfacial interaction force between the carbon substrate and the NiCo-LDH.The CC-NC-LDH composites feature three-dimensional open frameworks with vertically oriented NiCo-LDH nanosheets.The monolithic composite electrodes show a high specific capacity（1 817 F g-1@1 A g-1）and outstanding rate capability（60%@100 A g-1）.This nitrogen-doped carbon layer strategy can also be applied to different inert carbon-based substrate and active species,and realize the vertically assembly of active species on the substrate,which verify the universality of the superhydrophilic interface regulation.The phosphate species-enriched mesoporous phosphide on carbon fiber（Ni-CoPIP-O）have been fabricated via a low-temperature phosphorization strategy from the NiCo carbonate hydroxide and carbon fiber.The generation of the well-dispersed and high levels of phosphate species could be ascribed to the thermo-decomposition process during the phosphorization.In addition,the phosphate species are beneficial to the strong affinity of OH-in the electrolyte.The as-made Ni-CoPIP-O composites exhibit a superb rate capability（74%@50 A g-1）,which can be attributed to the one-dimensional nanowire with abundant mesopores and well-dispersed and high levels of phosphate species.The CH-Ni（OH）₂ composites consisted of NiCo carbonate hydroxide（CH）nanowires and Ni（OH）₂ microplates grown on the carbon fiber were used as research unit.We reveal that the CH nanowires in the CH-Ni（OH）₂ are experienced an in-situ electrochemical phase transition during the cyclic voltagmmetry（CV）cycling,and transformed to oxygen vancancies-enriched LDH nanosheets eventually.The operando X-ray absorption fine structure analysis,unveil that the irrevesible redox of Co cations at the early stage of the CV cycling induces the phase transition in the CH nanowires forming oxygen vacancies-enriched LDH nanosheets.Theoretical modeling demonstrates that the unsaturated 5-coordinated Co sites which produced in the CV cycling,exhibit optimal redox reaction energy barrier and beneficial to the reaction activity and reversible charge storage of the materials.After 150 CV cycles,the as-obtained oxygen vacancies-enriched composites exhibit a 10.5-fold increase in the specific capacity,and achieve a capacity retention of 86%even the current density became 100 times larger.The vacancies-enriched β-Ni（OH）₂ on carbon fiber have been fabricated via a electrochemical abuse strategy from NiV-LDH and carbon fiber.The experimental results reveal that the electrochemical cycling within high voltage window will results in the irreversible oxidation and dissolution of vanadium,which is the origin for the formation of unsaturated Ni sites,disorder phases and strain.The specific capacity of the vacancies-enrichedβ-Ni（OH）₂ composites is 457 C g-1 at current density of 30 A g-1,which is～2 times and～4 times those of the regular α-Ni（OH）₂ and β-Ni（OH）₂,respectively.Finite element analysis illustrates that the increase of disorder phase and defects during the electrochemical cycling results in more strain and the corrugated structure in the nanosheets.In addition,the corrugated nanosheets show more robust mechanical reliability than the flat nanosheets and contributes to the long-term cycling stability.